PHYSICAL REVIEW MATERIALS 2, 043402 (2018) Nitrogen grain-boundary passivation of In-doped ZnO transparent conducting oxide D. Ali, 1, 2 M. Z. Butt, 3 C. Coughlan, 2 D. Caffrey, 2, 4 I. V. Shvets, 2, 4 and K. Fleischer 2 1 Department of Physics, GC University Lahore-54000, Pakistan 2 School of Physics, Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland 3 Centre for Advanced Studies in Physics, GC University Lahore-54000, Pakistan 4 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland (Received 28 December 2017; published 9 April 2018) We have investigated the properties and conduction limitations of spray pyrolysis grown, low-cost transparent conducting oxide ZnO thin films doped with indium. We analyze the optical, electrical, and crystallographic properties as functions of In content with a specific focus on postgrowth heat treatment of these thin films at 320 ◦ C in an inert, nitrogen atmosphere, which improves the films electrical properties considerably. The effect was found to be dominated by nitrogen-induced grain-boundary passivation, identified by a combined study using in situ resistance measurement upon annealing, x-ray photoelectron spectroscopy, photoluminescence, and x-ray diffraction studies. We also highlight the chemical mechanism of morphologic and crystallographic changes found in films with high indium content. By optimizing growth conditions according to these findings, ZnO:In with a resistivity as low as 2 × 10 −3  cm, high optical quality (T ≈ 90%), and sheet resistance of 32/ has been obtained without any need for postgrowth treatments. DOI: 10.1103/PhysRevMaterials.2.043402 I. INTRODUCTION Transparent conducting oxides (TCOs) are a unique class of material that simultaneously possess both high optical transparency 85%–90% in the visible region and low electrical resistivity ∼10 −4  cm. TCOs are widely used in solar cells, light-emitting diodes, liquid-crystal displays, touch screen panels, and in some specific cases in gas sensors [1–7]. From a commercial standpoint the most widely utilized TCO is tin-doped indium oxide (ITO). The ubiquity of ITO can be attributed to its superior optical and electrical properties. However, high indium material costs, the need for sputter deposition, and its brittleness puts limits on its use in low- cost or flexible devices. As a result an alternative is highly desirable. To overcome these limitations, various alternative inexpensive as well as abundantly available materials have gained considerable attention. Among these various materials zinc oxide (ZnO) has emerged as one of the most promising candidates [8,9]. Intrinsic ZnO has a high optical transparency in the visible region, but a low electrical conductivity resulting from a low intrinsic carrier concentration. An effective way to improve ZnO electrical properties, without deteriorating the optical properties, is by doping with group-III elements (In, Al, and Ga). These materials substitute on the Zn site generating a shallow donor level, thereby increasing the carrier concentration [10,11]. In this work, we focus on indium doping, which has been found to be an effective means to improve electrical properties of ZnO [4,12–19]. Historically, the highest performance ZnO thin films have been prepared by physical vapor deposition (PVD) techniques such as sputtering [10,15,18] and pulsed laser deposition [20,21]. However, these processes are complicated by their dependence on high vacuum systems. On the other hand, thin films prepared by chemical vapor, or solution based methods (chemical bath deposition, sol-gel spin coating, and spray pyrolysis) are mechanically simple, can be performed at atmospheric pressure, facilitate large area coverage and simple composition control and therefore facilitate low production costs [19,22–27]. One of the major concerns with such low-cost methods is that the electrical conductivity of the produced ZnO typically suffers in comparison to best performing PVD prepared films. The lower performance of chemical vapor deposition (CVD) produced films typically originates from a combination of crystalline quality, a high number of grain boundaries, and the formation of a hydroxide/double hydrox- ide layer on the top and grain surface. To improve the electrical conductivity of ZnO films can be irradiated with ultraviolet (UV) light [28–30] or postannealed in reducing environments [28,31–33]. Several physical effects can contribute to conductivity changes in postgrowth treated ZnO including recrystallization, dephasing, generation, and reordering of extrinsic and intrinsic point defects and passivation of charge traps at grain bound- aries. This creates a complex interplay of effects which are currently not well understood at a fundamental level, even if certain behaviors can be anticipated. The main objective of the present work is to decouple these effects by specifically investigating the passivation of grain boundaries using low- temperature nitrogen annealing. By containing this study to films annealed at temperatures substantially below the initial growth temperature, we exclude crystallographic changes upon annealing, allowing us to solely focus on the effects of the nitrogen passivation. The effect of the nitrogen annealing on the grain boundaries is discussed for a wide range of doping levels from 0.2% to 10%. Using the optimum dopant levels and N 2 postannealing, we demonstrate that ZnO:In with a resistivity as low as 5 × 10 −3  cm and a Hall mobility up to 7 cm 2 V −1 s −1 2475-9953/2018/2(4)/043402(10) 043402-1 ©2018 American Physical Society